::inheritance

This (experimental) crate provides procedural macros to get "inheritance-like" behavior by deriving delegation from composition.

Presentation

Imagine having some Point type and some behavior / associated methods:

#[derive(Debug, Clone, Copy, PartialEq)]
struct Point {
    x: f32,

    y: f32,
}

impl Point {
    fn x (self: &'_ Self) -> f32
    {
        self.x
    }

    fn y (self: &'_ Self) -> f32
    {
        self.y
    }

    fn name (self: &'_ Self) -> Option<&'_ str>
    {
        Some("Point")
    }
}

Now imagine you want to have a new "Point-like" type, but with extra attributes (and maybe some overriden behavior):

# struct Point;
struct NamedPoint {
    name: String,

    point: Point,
}

First of all, being "Point-like" requires abstracting the inherent methods under a trait:

#[derive(Debug, Clone, Copy, PartialEq)]
struct Point {
    x: f32,

    y: f32,
}

trait IsPoint {
    fn x (self: &'_ Self) -> f32;

    fn y (self: &'_ Self) -> f32;

    fn name (self: &'_ Self) -> Option<&'_ str>;
}

impl IsPoint for Point {
    fn x (self: &'_ Self) -> f32
    {
        self.x
    }

    fn y (self: &'_ Self) -> f32
    {
        self.y
    }

    fn name (self: &'_ Self) -> Option<&'_ str>
    {
        Some("Point")
    }
}

Now we'd like NamedPoint to implement IsPoint:

# #[derive(Debug,Clone,Copy,PartialEq)]struct Point{x:f32, y:f32}trait IsPoint{fn x(&self)->f32;fn y(&self)->f32;fn name(&self)->Option<&str>;}impl IsPoint for Point{fn x(&self)->f32{self.x}fn y(&self)->f32{self.y}fn name(&self)->Option<&str>{Some("Point")}}
struct NamedPoint {
    name: String,

    point: Point,
}

impl IsPoint for NamedPoint {
    #[inline]
    fn x (self: &'_ Self) -> f32
    {
        self.point.x()
    }

    #[inline]
    fn y (self: &'_ Self) -> f32
    {
        self.point.y()
    }

    #[inline]
    fn name (self: &'_ Self) -> Option<&'_ str>
    {
        self.point.name()
    }
}

This leads to writing very repetitive and uninteresting (delegation) code...

Enter ::inheritance

This last implementation can be completely skipped by slapping a #[inheritable] attribute on the IsPoint trait, and a Inheritance derive on the NamedPoint struct:

use ::inheritance::{inheritable, Inheritance};

#[derive(Debug, Clone, Copy, PartialEq)]
struct Point {
    x: f32,

    y: f32,
}

#[inheritable]
trait IsPoint {
    fn x (self: &'_ Self) -> f32;

    fn y (self: &'_ Self) -> f32;

    fn name (self: &'_ Self) -> Option<&'_ str>;
}

impl IsPoint for Point {
    fn x (self: &'_ Self) -> f32
    {
        self.x
    }

    fn y (self: &'_ Self) -> f32
    {
        self.y
    }

    fn name (self: &'_ Self) -> Option<&'_ str>
    {
        Some("Point")
    }
}

#[derive(Inheritance)]
struct NamedPoint {
    name: String,

    #[inherits(IsPoint)]
    point: Point,
}

nightly Rust

This feature is especially interesting with the specialization feature, which you can already use in nightly.

When depending on the inheritance crate in your Cargo.toml, you can specify that you want to use this feature:

[dependencies]
inheritance = { version = "...", features = ["specialization"] }

You will then be able to override some of the auto-generated delegation methods:

# use::inheritance::{inheritable, Inheritance};#[derive(Debug,Clone,Copy,PartialEq)]struct Point{x:f32, y:f32}#[inheritable]trait IsPoint{fn x(&self)->f32;fn y(&self)->f32;fn name(&self)->Option<&str>;}impl IsPoint for Point{fn x(&self)->f32{self.x}fn y(&self)->f32{self.y}fn name(&self)->Option<&str>{Some("Point")}}
#[derive(Inheritance)]
struct NamedPoint {
    name: String,

    #[inherits(IsPoint)]
    point: Point,
}

# #[cfg(feature = "specialization")]
impl IsPoint for NamedPoint {
    fn name (self: &'_ Self) -> Option<&'_ str>
    {
        Some(&*self.name)
    }
}

Going further

Newtypes

The #[inherits(...)] field attribute can be used on tuple structs fields, which makes it the perfect tool for newtypes:

# use::inheritance::{inheritable, Inheritance};#[derive(Debug,Clone,Copy,PartialEq)]struct Point{x:f32, y:f32}#[inheritable]trait IsPoint{fn x(&self)->f32;fn y(&self)->f32;fn name(&self)->Option<&str>;}impl IsPoint for Point{fn x(&self)->f32{self.x}fn y(&self)->f32{self.y}fn name(&self)->Option<&str>{Some("Point")}}
#[derive(Inheritance)]
struct AnonymousPoint (
    #[inherits(IsPoint)]
    Point,
);

# #[cfg(feature = "specialization")]
impl IsPoint for AnonymousPoint {
    fn name (self: &'_ Self) -> Option<&'_ str>
    {
        None
    }
}

Multiple "inheritance"

A single struct with a #[derive(Inheritance)] on it can have multiple #[inherits(...)] on either the same field or multiple distinct fields, since the implementation of each "inherited" trait will just be delegating the methods of that trait onto the decorated field. If you try to "inherit" the same trait from multiple fields, then classic coherence rules will prevent it:

# use::inheritance::{inheritable, Inheritance};#[derive(Debug,Clone,Copy,PartialEq)]struct Point{x:f32, y:f32}#[inheritable]trait IsPoint{fn x(&self)->f32;fn y(&self)->f32;fn name(&self)->Option<&str>;}impl IsPoint for Point{fn x(&self)->f32{self.x}fn y(&self)->f32{self.y}fn name(&self)->Option<&str>{Some("Point")}}
#[derive(Debug, Clone, Copy, PartialEq)]
enum Color {
    Red,
    Green,
    Blue,
}

#[inheritable]
trait Colored {
    fn color (self: &'_ Self) -> Color;
}

impl Colored for Color {
    #[inline]
    fn color (self: &'_ Self) -> Color
    {
        *self
    }
}

#[derive(Inheritance)]
struct ColoredPoint {
    #[inherits(IsPoint)]
    point: Point,

    #[inherits(Colored)]
    pigments: Color,
}

Virtual dispatch

The idea behind "virtual" methods, à la C++, is that objects always always carry a vtable with some methods in it, the ones marked virtual (and with the other methods never in it). In Rust, however, "objects" sometimes carry a vtable (i.e., structs and enums do not, but trait objects (dyn Trait) do), and this vtable contains all the methods of the trait and its supertraits:

Imagine having a .present() method whose body calls .name(), and where the behavior / value returned by the .name() call changes for each different Point.

/// somewhere in the code
fn present (self: &'_ Self) -> Cow<'static, str>
// where Self : IsPoint
{
    if let Some(name) = self.name() {
        Cow::from(format!("{}({}, {})", name, self.x(), self.y()))
    } else {
        Cow::from("<anonymous>")
    }
}

let point = Point { x: 42., y: 27. };
assert_eq!(
    &point.present() as &str,
    "Point(42.0, 27.0)"
);

let anonymous_point = AnonymousPoint(point);
assert_eq!(
    &anonymous_point.present() as &str,
    "<anonymous>"
);

let named_point = NamedPoint { name: "Carré", point };
assert_eq!(
    &named_point.present() as &str,
    "Carré(42.0, 27.0)"
);

• In a language such as C++, this can be achieved by tagging the .name() method as virtual, and by then having .present() be a method of the base class (not necessarily virtual itself).

• In Rust the above strategy cannot be done; one must use a function / method polymorphic over implementors of the IsPoint type, using:

  • dynamic dispatch (the mechanism in C++ behind virtual methods):

    # use::inheritance::{inheritable, Inheritance};#[derive(Debug,Clone,Copy,PartialEq)]struct Point{x:f32, y:f32}#[inheritable]trait IsPoint{fn x(&self)->f32;fn y(&self)->f32;fn name(&self)->Option<&str>;}impl IsPoint for Point{fn x(&self)->f32{self.x}fn y(&self)->f32{self.y}fn name(&self)->Option<&str>{Some("Point")}}use::std::borrow::Cow;
    impl dyn IsPoint + '_ {
        fn present (self: &'_ Self) -> Cow<'static, str>
        {
            if let Some(name) = self.name() {
                Cow::from(format!("{}({}, {})", name, self.x(), self.y()))
            } else {
                Cow::from("<anonymous>")
            }
        }
    }

  • static dispatch; then getting method-like syntax requires a helper trait:

    # use::inheritance::{inheritable, Inheritance};#[derive(Debug,Clone,Copy,PartialEq)]struct Point{x:f32, y:f32}#[inheritable]trait IsPoint{fn x(&self)->f32;fn y(&self)->f32;fn name(&self)->Option<&str>;}impl IsPoint for Point{fn x(&self)->f32{self.x}fn y(&self)->f32{self.y}fn name(&self)->Option<&str>{Some("Point")}}use::std::borrow::Cow;
    trait Present
    where
        Self : IsPoint,
    {
        fn present (self: &'_ Self) -> Cow<'static, str>
        {
            if let Some(name) = self.name() {
                Cow::from(format!("{}({}, {})", name, self.x(), self.y()))
            } else {
                Cow::from("<anonymous>")
            }
        }
    }
    impl<T : ?Sized> Present for T
    where
        T : IsPoint,
    {}

Currently the macro does not target full-featured "virtual" methods, so overriding .present() itself is not yet possible.

Since this is an experimental crate, I will try to explore that possibility...